SAW devices (which may also be referred to as “SAW filters”) are successfully used in wireless communication systems as a result of their small size and low insertion loss provided by resonator-type structures, built on piezoelectric substrates with high electromechanical coupling factor. Such devices commonly utilize low-attenuated quasi-bulk leaky surface acoustic waves (LSAW) characterized by a high electromechanical coupling factor. Such waves are known to exist in two piezoelectric crystals belonging to the same 3m symmetry class. The known crystals are lithium tantalate, LiTaO3 (LT), and lithium niobate, LiNbO3 (LN).
Recently, devices fabricated using non-leaky quasi-shear horizontal wave devices have been reported. The suppression of the leakage effect results when the surface on which the acoustic wave propagates is sufficiently loaded with, e.g., electrodes and/or dielectric overcoat, so as to slow the wave velocity such that its value is less than that of the slow quasi-shear bulk wave propagating in the same direction.
Known SAW devices include heavy metal electrodes, deposited on a Y-rotated, X-propagating lithium niobate (YX-LN) substrate, and buried beneath a dielectric overcoat with a positive temperature coefficient of frequency (TCF). These devices may suffer from challenges that are not found with respect to devices fabricated on orientations of Y-rotated, X-propagating lithium tantalate (YX-LT) substrates. For example, a second acoustic wave mode is found that propagates on the surface of YX-LN substrates that can produce significant spurious responses in the filter's passband.
The performance characteristics of a conventional SAW device may depend predominately upon its electromechanical coupling factor, the attenuation of propagation, the TCF, and the absence of spurious acoustic wave modes. In YX-LN substrates, two SAW modes can propagate with different velocities and different electromechanical coupling coefficients. In a SAW device utilizing the SAW mode with higher coupling, the SAW mode with lower coupling produces spurious response. The performance of SAW devices may, therefore, depend upon the degree by which the spurious response is suppressed. Hereinafter, for convenience of description, high-coupling mode may be referred to as the “desired mode,” and the low-coupling mode may be referred to as the “spurious mode.”
Polarizations of the two SAW modes may depend on a substrate's rotation angle, mechanical load (which may depend on characteristics of electrodes on surface of substrate), and electrical boundary condition. The polarization components may generally include a longitudinal (L) component, a shear horizontal (SH) component, and a shear vertical (SV) component. A contribution of the SH-component or a combination of the L and SV components may vanish for certain rotation angles, which means that corresponding combination of the L and SV components or the SH component can propagate in such selected orientations. This may complicate use of polarization to distinguish between the two SAW modes for arbitrary rotation angles and electrode thicknesses (or masses).
Thus, for an arbitrary rotation angle and electrode thickness (or mass) it may not be possible to distinguish between these two SAW modes by examining their polarizations. Instead, their propagation velocities may be used. Alternatively, the SAW mode with a substantially greater coupling factor may be qualified as the “desired SAW mode” and the weaker SAW mode as the “spurious SAW mode”.
For an unloaded surface, i.e., a surface without electrodes disposed thereon, one of the two SAW modes propagating in a YX-LN substrate will be slower than a slow shear bulk wave propagating in the same substrate, independent of substrate rotation angle. A contribution of an SH-component into polarization of this relatively slow-moving SAW mode may be small, for any propagation angle, when the surface is unloaded. This relatively slow-moving SAW mode, having a horizontal shear component that is small as compared to its vertical shear and longitudinal components, may sometimes be referred to as a Rayleigh-type wave.
The velocity of the relatively fast-moving SAW mode lies between that of the slow shear and fast shear bulk waves, when the surface is unloaded, thus making this wave leaky. It becomes ‘non-leaky’, when the surface is sufficiently loaded. The relatively fast-moving SAW mode may sometimes be referred to as a Love-type wave. A Love-type wave may include a relatively greater SH component, as compared to a Rayleigh-type wave, though it is not SH-polarized in general.
While extensive efforts have been made to improve performance of SAW devices, there remains a need for SAW devices to simultaneously exhibit strong electromechanical coupling, low spurious responses, and desired frequency-temperature characteristic for application in radio frequency (RF) filtering for wireless communications.